A Partial Restoration of a Heathkit HW-32A

With the HP-23 power supply working, I was ready to bring my HW-32A transceiver back to life. I bought it together with a home-brewed power supply and a Heathkit SWR meter when I was a teenager in the early 1980s (more precisely, my mother bought it for me). The transceiver was originally a kit, but I bought it built. I used it extensively for a few year, after which it spent a few decades in a box. The home-brewed power supply seemed reasonably decent on the outside, but was a scary mess on the inside I did not dare plug it in (after my teenage years, that is), but the HP-23 solved the problem.

IMG_5862

The transceiver was pretty clean on the inside. It was modified in a couple of ways, some of which I knew about and some that I did not know about. On top of the enclosure I installed a large 115VAC fan, to keep the finals cool. It was ugly and the transceiver was designed to operate without a fan, so I removed it. I was happy to discover that I attached it with thin screws that went through the perforation in the enslosure; I did not drill holes or enlarge them. The fan is not shown in the picture above, but you can see another mod: the little unetched PCB facing up on the back. This is IMG_9683a receive preamplifier that I build (and completely forgot about). It was plugged into the calibrator socket in the transceiver. The socket provides it with 12VAC  feeds back the amplified signal to the receiver. I think the transistor is a dual gate mosfet, but I didn’t check carefully. I removed this mod, both because it was mechanically unsound and because I wanted to get the transceiver back to a close to original condition.

The transceiver used three electrolytic capacitors which I replaced before powering it up. With these replaced, I powered up the transceiver and started testing it. The receiver worked fine, but was about 100kHz off frequency. The adjustment for that is a coil and once I borrowed a suitable plastic tuning tool for it, I was able to align the frequency pretty accurately. The transmitter also seemed to be in a decent shape and produced some output. After some testing, however, the receive-transmit relay stopped switching. I am not sure what happened. I think I operated the transceiver for a while on the 300V setting of the HP-23 rather than on the 250V setting. This might have damaged some of the tubes; I’m not exactly sure what happened. The relay did not seem completely dead, however. It is an open-frame relay, and by applying a bit of force I was able to get it to switch. The voltages across it seemed reasonable. I eventually resorted to adding a 10uF electrolytic capacitor in parallel with R28, a 12K resistor that is connected in series with the relay. In receive mode (the relay is not energized), the capacitor is discharged. When the triode turns on to activate the relay, current starts to flow but the voltage across the capacitor is still close to zero, so the relay sees a larger voltage than it does when connected in series with the resistor. As the capacitor discharges, the voltage across the relay drops, but it remains large enough to hold the relay. When the triode turns off, the relay drops out almost instantaneously. Problem solved.

hw-32a-relay-circuitWhen inspecting the bottom side of the circuit board I noticed another mod: a white-blue wire was attached to a white-yellow wire on the other side of the chasis by a piece of black wire. This could not be part of the original circuit, because Heathkits never had different-color wires connected together. I looked up these wires by color in the assembly manual and eventually discovered that the white-blue one carries signal 26 in the diagram above. It grounds C105 to allow voice-activated T/R switching. In my transceiver it grounded instead the white-yellow wire, which turned out to be connected to the amplifier T/R switching pin on the power connector. The white-yellow wire was supposed to get grounded during transmit by a pole of the relay. At this point I noticed that the relay in my transceiver was a DPDT one; the manual shows a 3PDT relay. There was no pole to switch the amplifier, so somebody hooked this wire up to be grounded in VOX and Tune modes. My transceiver used this switched signal to turn on the fan, not an amplifier. I left this mod in since there was no way to switch an amplifier anyway. Also, one of the terminals of the neon lamp in the VOX circuit is broken in my transceiver, so VOX is not working anyway. Not grounding poing 26 has no effect anyway.

SAMSUNGI had one more difficulty with the transceiver. The successful tuning of the VFO frequency encouraged me to follow the rest of the tuning procedures in the manual. Some of them involved tuning the cores (called slugs in the manual) of coils and transformers. Some of them were almost stuck, but I was able to gently free them and tune them. As far as I remember, none was really off. One of them, however, was very much stuck.  tried to turn the core, but at some point the stuck core caused the entire coil to turn, and this broke one of the the terminal connections (the coil was inside an aluminum shield, but I could feel that something turned and tore, and a multimeter check showed that one side of the transformer was an open circuit. Fortunately, these transformers are so large that it was easy to solder it out of the circuit board, open it up, solder the torn wire, and put everything back together. It was a good lesson in just how repairable these transceivers are.

I did a bit of tracing with an RF probe and with a scope to try to figure out why the output was so low. I was not able to pinpoint the problem and since at this point the SB-101 was in a much better shape, I decided to leave the HW-32 alone. It is now functional and in close to original condition. The receiver seems okay (I didn’t check sensitivity, but it certainly picks up signals, even weak ones). The transmitter works but produces very little power. It will do for now.

IMG_5863

In this final picture you can see the transceiver with the Motorola microphone it came with. It uses the original Heathkit 2-pin connector. The speaker is the one I used with the transceiver years ago and it still works fine. It has a headphone socket (that does not mute the speaker) and a separate mute switch. Pretty useful. The red paint is my work from many years ago. I think I used red because simply because we had a can of red paint at home back then.

 

Advertisements